Memory controller having precharge prediction based on processor and PC bus cycles

ABSTRACT

A memory controller which provides a series of queues between the processor and the PCI bus and the memory system. Memory coherency is maintained in two different ways. Before any read operations are accepted from the PCI bus, both of the posting queues must be empty. A content addressable memory (CAM) is utilized as the PCI to memory queue. When the processor performs a read request, the CAM is checked to determine if one of the pending write operations in the PCI to memory queue is to the same address as the read operation of the processor. If so, the read operation is not executed until the PCI memory queue is cleared of the write. To resolve the problem of aborting a Memory Read Multiple operation, an abort signal from the PCI bus interface is received and as soon thereafter as can be done the read ahead cycle is terminated, even though the read ahead cycle has not fully completed. The memory controller has improved prediction rules based on whether the cycle is coming from the processor or is coming from the PCI bus to allow more efficient precharging when PCI bus cycles are used. The memory controller is highly programmable for multiple speeds and types of processors and several speeds of memory devices. The memory controller includes a plurality of registers that specify number of clock periods for the particular portions of a conventional DRAM cycle which are used to control state machine operations.

This is a continuation of application Ser. No. 08/324,112, filed Oct.14, 1994, now U.S. Pat. No. 5,634,112.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to memory controllers utilized with computersystems, and more particularly to memory controllers which control DRAMsand have idle states between memory cycles.

2. Description of the Related Art

Computer systems are becoming ever more powerful by the day. Users arerequiring more capabilities to run ever more complicated andsophisticated applications and computer system manufacturers areresponding. Computer speeds have dramatically increased over the lastnumber of years so that now desktop and file server computers canreadily outperform mainframe computers of 10-15 years ago. But the questfor further performance is never ending. To this end, the microprocessormanufacturers have been developing ever faster microprocessors.

However, a computer system is far more than just a microprocessor. Thereare many other subsystems that must cooperate with the microprocessor toprovide a complete computer system. It is desirable to optimize as manyof these subsystems as possible and yet take into account cost andsystem flexibility to satisfy varied user desires.

Two of the subsystems which have not maintained pace with thedevelopment of microprocessor are the main memory systems and theinput/output buses. Main memory system shortcomings have been muchalleviated by the use of cache memory systems, but in the end all memoryoperations must ultimately come from the main memory, so that itsperformance is still a key piece in the overall performance of thecomputer system. Many advanced memory architectures and techniques havedeveloped over the years. One of the most common techniques is the useof paged mode memory devices or DRAMS, where the actual memory addresslocation value is divided into rows and columns, and if the row address,i.e., the page, is the same for the subsequent operation, only columnaddresses need to be provided to the DRAM. Although there is a certainamount of overhead required, it easily pays for itself by the improvedperformance gained during a page hit. So basic page mode operationprovides a major performance increase, but more performance is alwaysdesired.

One further performance increase relates to an improvement fordetermining the level of the row address strobe or RAS* signal when thememory system is idle. As is well known, the RAS* signal must be negatedor set high to allow a new page or row address to be provided and thereis also a precharge time requirement. Thus, there is a performancepenalty if the RAS* signal is raised when the next operation is actuallya page hit. Similarly, there is a delay if the RAS* signal is kept lowand the operation is a page miss, as the full precharge time must alsobe expended after the cycle has been issued. To address this concern,various techniques have been developed to predict whether the RAS*signal should be kept low or should be returned high to indicate a newpage cycle. The prediction can be done several ways, as indicated inSer. No. 08/034,104 filed Mar. 22, 1993, entitled Memory Controller ThatDynamically Predicts Page Misses. In that application several techniquesare used. A first, simple technique bases the prediction on the type ofthe last cycle performed by the processor, with the choice always fixed.A second, more sophisticated technique samples the hits and misses foreach cycle type and then sets the RAS* level based on this adaptivemeasurement. But the techniques have been based on the use of theprocessor cycles and have not based themselves on the I/O bus cycles.Therefore, I/O bus master operations still performed at lesser levels.

One high performance I/O bus is the PCI or Peripheral ComponentInterconnect bus developed by Intel Corp. and accepted by many computermanufacturers. PCI is a high performance bus and allows numerous busmasters to be present. The bus masters are essentially local processorswhich perform specific duties, not general processing duties. By havingthese bus masters, the main processor is able to off load variousspecialized processing tasks, so that more tasks can be performed inparallel, thereby increasing the performance of the computer system.This is but one example of how parallelism is being used in currentcomputer systems.

It is desirable to have as many operations running in parallel orconcurrently as possible to allow increased overall performance. One waythis concurrent operation has been done in the past is by the use ofwrite posting, where a single cycle from the processor is latched into aposting buffer and ready is returned to the processor prior to the writecycle actually being completed to the memory or I/O device. The entiredata and address values are posted in a latch and then the cycleexecuted on the target bus when possible. However, write posting hasbeen kept at a very simple level, such as one level per bus, because ofcomplications in memory coherency and cycle ordering which result ifdeeper posting were to be performed. Therefore, it can be seen thatthere are numerous gains that could be obtained if one were able towrite post more than a single operation to a given bus, if thecomplications could be simply solved.

The PCI bus provides opportunities to increase overall systemperformance, particularly that of the memory system. One of the readoperations defined for the PCI bus is what is termed as a Memory ReadMultiple cycle, which is used to indicate a desire to read a number ofcache or memory lines, not just a single line. As noted in the PCI buscycle definition, this cycle decoding provides an opportunity for thememory controller to start doing read aheads or pipelining so that thedata can be obtained prior to actually being required on the PCI bus.With this data then obtained, the memory controller can allow access bythe processor, thus further increasing overall system concurrency.However, it is also common for PCI bus masters to abort cycles prior totheir completion, and if such an aborted cycle were to occur shortlyafter a Memory Read Multiple cycle has commenced, then a read aheadoperation would have been started and would conventionally complete,only to have the data then immediately discarded. This would reduceoverall system performance because of the wasted operations needed tostart and complete the full read ahead operation. Therefore, it would bedesirable to reduce the wasted time when doing read aheads during MemoryRead Multiple cycles when the cycle is aborted early by the bus masteron the PCI bus.

Further, personal computer systems are becoming mass market products,and therefore need to be very flexible to meet the widely varyingparticular goals of users. For example, some users may desire theultimate in performance with little regard for cost, whereas other usersmay be significantly more cost sensitive. One area where cost directlyimpacts performance is in the speed of the memory devices used in themain memory. Another area of impact is the economies of scale whichcould be obtained by using a single memory controller chip for manydifferent microprocessor configurations and speeds. But using a singlememory controller usually involves performance tradeoffs. The memorycontroller as disclosed in U.S. Pat. No. 5,333,293 addressed themultiple speed processor point, but could use only a single speed ofmemory devices, thus limiting user options and performance tradeoffs. Inanother memory controller as disclosed in Ser. No. 08/034,290 filed Mar.22, 1993, the memory controller can handle different speed memorydevices on a bank-by-bank or module-by-module basis, and yet allowsoptimal timing for each particular memory device. However, this memorycontroller was designed to be used with a single processor operating ata single speed, thus providing user flexibility but not economy ofscale. It would be more desirable to allow numerous types and speeds ofprocessors to be utilized with a single memory controller, and yet allowuse of numerous types and speeds of memory devices without requiringgreat complexity.

SUMMARY OF THE INVENTION

A computer system according to the present invention has a memorycontroller that provides numerous performance increases, particularly inthe PCI bus environment, and can readily work with numerous types andspeeds of processors and different speed memory devices.

The memory controller provides a series of queues between the processorand the PCI bus and the memory system to allow deep write posting. Inthe preferred embodiment, four quadword addresses can be posted from theprocessor and eight quadword addresses from the PCI bus for writeoperations. Memory coherency is maintained in two different ways. Beforeany read operations are accepted from the PCI bus, both of the postingqueues must be empty. In this way, all writes are completed prior to theread occurring, so that the main memory is coherent for the readoperation from the PCI bus. However, more performance is desired fromthe processor, and therefore to maintain coherency a content addressablememory (CAM) is utilized as the PCI to memory queue. When the processorperforms a read request, the CAM is checked to determine if one of thepending write operations in the PCI to memory queue is to the sameaddress as that read operation of the processor. If so, the readoperation is not executed until the PCI memory queue has cleared thatentry. If no address hit occurs, the read operation is accepted andexecuted according to arbitration priority rules. Again, in this manner,the main memory is coherent prior to the read operation occurring. It isnoted that allowing two write operations to the same address to bepresent in the two queues is not a problem and does not produceincoherent results, as the exact timing between the buses would never beclear in any event.

In the preferred embodiment the PCI bus capability of read aheadoperations when a Memory Read Multiple has been requested is present.This allows the memory system to obtain data at a high rate and leave itposted for reading by the PCI bus master when indicated by theparticular cycle. However, as noted in the background, it is possiblethat the PCI bus master would abort the cycle prior to its completion.To resolve this problem, a memory controller according to the preferredembodiment receives an abort signal from the PCI bus interface and assoon thereafter as can be done, while maintaining DRAM data integrity,terminates the read ahead cycle, even though the read ahead cycle hasnot fully completed. Thus, the read ahead cycle is aborted as soon aspossible. Therefore, the full read ahead does not occur, so that thesituation of an abort occurring during a read ahead operation does notoverly hinder performance as would normally be the case.

To further improve the system, the memory controller of the preferredembodiment has improved prediction rules for determining when toprecharge the DRAM devices. The prediction rules are based on whetherthe cycle is coming from the processor or is coming from the PCI bus. Byusing these new rules, more efficient precharging is done, andadditionally, more page mode cycles can be performed than otherwisewould have been done according to the prior art.

Finally, the memory controller of preferred embodiment is highlyprogrammable for multiple speeds and types of processors and severalspeeds of memory devices, and yet can be simply programmed. The memorycontroller includes a plurality of registers that specify the number ofclock periods for the particular portions of a conventional DRAM cycle,such as the address set up and hold times, CAS* signal pulse width, theprecharge time and the data set up time. These registers are thusconsistent with the normal timing concerns and parameters of DRAMdevices, so that the designer need only know the particular processortype and clock speed and memory device speed and then the registers canbe properly and simply programmed from a small matrix or table. Complexoperations and alternatives are not necessary and the clock periodvalues can be easily determined. By the use of the single memorycontroller for multiple processor of types and speeds and plural speeds,the economies of scale can now be obtained by increasing manufacturingvolumes while still allowing user flexibility.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 is a block diagram of a system board of a computer systemaccording to the preferred embodiment;

FIG. 2 is a block diagram of a processor board according to the presentinvention for use with the computer system of FIG. 1;

FIG. 3 is a block diagram of a video system for use with the computersystem of FIG. 1;

FIG. 4 is a block diagram of the memory controller of the processorboard of FIG. 2;

FIG. 5 is a block diagram of the data buffer of FIG. 2;

FIG. 6 is a block diagram of the circuitry used to provide memory timinginformation and addresses for a particular memory cycle in the memorycontroller of FIG. 4;

FIGS. 7 and 8 are block diagrams of portions of the memory controller ofFIG. 4;

FIG. 9 is a block diagram of the CAM hit detection logic of the memorycontroller of FIG. 4; and

FIG. 10 is a state machine diagram of the memory state machine;

FIG. 11 is a state machine diagram of the memory address state machine;

FIG. 12 is a state machine diagram of the memory data state machine;

FIG. 13 is a state machine diagram of the precharged state machine;

FIG. 14 is a state machine diagram of the memory to I/O state machine;and

FIG. 15 is a state machine diagram for the memory to PCI next addressstate machine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The following disclosures are hereby incorporated by reference:

U.S. application Ser. No. 08/324,016, entitled Single Bank, Multiple WayCache Memory, by Sompong O. Olarig, Jens K. Ramsey, and Michael J.Collins, filed Oct. 14, 1994;

U.S. application Ser. No. 08/324,246, issued May 27, 1997 as U.S. Pat.No. 5,634,073, entitled Memory Controller With Write Posting Queues forProcessor and I/O Bus Operations and Ordering Logic for Controlling theQueues, by Michael J. Collins, Gary W. Thome, Michael Moriarty, Jens K.Ramsey, and John E. Larson, filed Oct. 14, 1994;

U.S. application Ser. No. 08/323,263, issued Sep. 10, 1996 as U.S. Pat.No. 5,555,250, entitled Data Error Detection and Correction System, byWilliam J. Walker and Alan L. Goodrum, filed Oct. 14, 1994;

U.S. application Ser. No. 08/324,020, entitled Circuit for InvalidatingPortions of a Cache Memory if a Write Occurs to a Write Protected Areaof Main Memory by Jens K. Ramsey, filed Oct. 14, 1994;

U.S. application Ser. No. 08/323,110, entitled Circuit for Placing aCache Memory Into Low Power Mode in Response to Special Bus Cycles, byJens K. Ramsey, and Jeffrey C. Stevens, filed Oct. 14, 1994; and

U.S. application Ser. No. 08/324,011, issued Jun. 4, 1996 as U.S. Pat.No. 5,524,235, entitled System for Arbitrating Access to Memory by JohnE. Larson, Michael Moriarty, Michael J. Collins and Gary W. Thome, filedOct. 14, 1994; all of which are assigned to the assignee of thisinvention.

Referring now to FIG. 1, the system board S of a computer systemaccording to the present invention is shown. In the preferredembodiment, the system boards contain circuitry and slots for receivinginterchangeable circuit boards. In the preferred embodiment, there aretwo primary buses located on the system board S. The first bus is thePCI or Peripheral Component Interconnect bus 98 which includesaddress/data portion 100, control and byte enable portion 102 andcontrol signal portion 104. The second primary bus on the system beard Sis the EISA bus E. The EISA bus E includes LA address portion 106, SAaddress portion 108, SD data portion 110 and EISA/ISA control signalportion 112. The PCI and EISA buses P and E form the backbones of thesystem board S.

A CPU connector 114 is connected to the PCI bus 98 to receiveinterchangeable processor cards, such as the one shown in FIG. 2. A PCIgraphics connector 116 is connected to the PCI bus 98 to receive a videographics card as shown in FIG. 3. A PCI option connector 118 is alsoconnected to the PCI bus 98 to receive any additional cards designedaccording to the PCI standard. In addition, a SCSI and network interface(NIC) controller 120 is connected to the PCI bus 98. Preferably thecontroller 120 is a single integrated circuit and includes thecapabilities necessary to act as a PCI bus master and slave and thecircuitry to act as a SCSI controller and an Ethernet interface. A SCSIconnector 122 is connected to the controller 120 to allow connection ofvarious SCSI devices, such as hard disk drives and CD-ROM drives. AnEthernet connector 124 is provided on the system board S and isconnected to filter and transformer circuitry 126, which in turn isconnected to the controller 120. This forms a network or Ethernetconnection for connecting the system boards and computer to a local areanetwork (LAN).

A PCI-EISA bridge 130 is provided to convert signals between the PCI bus98 and the EISA bus E. The PCI-EISA bridge 130 includes the necessaryaddress and data buffers and latches, arbitration and bus master controllogic for the PCI bus, EISA arbitration circuitry, and EISA buscontroller as conventionally used in EISA systems and a DMA controller.Preferably the PCI-EISA bridge 130 is single integrated circuit, butother combinations are possible. A miscellaneous system logic chip 132is connected to the EISA bus E. The miscellaneous system logic chip 132contains a digital audio interface, counters and timers asconventionally present in personal computer systems, an interruptcontroller for both the PCI and EISA buses P and E and power managementlogic, as well as other miscellaneous circuitry. A series of four EISAslots 134 are connected to the EISA bus E to receive ISA and EISAadapter cards. A combination I/O chip 136 is connected to the EISA busE. The combination I/O chip 136 preferably includes a floppy diskcontroller, real time clock (RTC)/CMOS memory, two UARTs, a parallelport and various address decode logic. A floppy disk connector 138 forreceiving a cable to a floppy disk drive is connected to the combinationI/O chip 136. A pair of serial port connectors are also connected to thecombination I/O chip 136, as is a parallel port connector 142. A buffer144 is connected to both the EISA bus E and the combination I/O chip 136to act as a buffer between the EISA bus E and a hard disk driveconnector 146 to allow connection of an IDE-type hard disk drive. Anon-volatile random access memory (NVRAM) 148 is connected to the EISAbus E and receives its control signals from the combination I/O chip136. An address latch 150 is connected to the EISA bus E and controlledby the combination I/O chip 136 to provide additional addressingcapability for the NVRAM 148. Preferably the NVRAM 148 is used tocontain certain system information. A data buffer 152 is connected tothe SD portion of the EISA bus E to provide an additional data bus XDfor various additional components of the computer system. The NVRAM 148is connected to the XD data bus to receive its data bits. A flash ROM154 receives its control and address signals from the EISA bus E and isconnected to the XD bus for data transfer. Preferably the flash ROM 154contains the BIOS information for the computer system and can bereprogrammed to allow for revisions of the BIOS. An 8742 or keyboardcontroller 156 is connected to the XD bus and EISA address and controlportions 108 and 112. The keyboard controller 156 is of conventionaldesign and is connected in turn to a keyboard connector 158 and a mouseor pointing device connector 160.

The computer system of the preferred embodiment also includes audiocapabilities. To this end a CODEC chip 162 is connected to themiscellaneous system logic chip 132 and to an analog amplifier and mixerchip 164. An FM synthesizer chip 166 is connected to the analogamplifier and mixer 164 and receives digital information from the XDbus. The FM synthesizer 166 is also connected to the control and dataportions 110 and 112 of the EISA bus E and is controlled by themiscellaneous system logic chip 132. An audio connector 168 is providedto allow external audio connections to the computer and is connected tothe outputs and inputs of the analog amplifier and mixer 164.

Referring now to FIG. 2, the processor board P is shown. In theprocessor board P of FIG. 2, the CPU or processor 200 can be any of aplurality of processors, such as the 486DX/33, 486DX2/66, 486DX4/50-100,486DX4/33-100, 486DX4/33-83, P24T, Pentium 50/75, Pentium 60/90, andPentium 66/100, and other similar and compatible processors. Theprocessor 200 provides data, address and control portions 202, 204 and206 to form a processor bus PB. A level 2 (L2) or external cache memorysystem 208 is connected to the processor bus PB to provide additionalcaching capabilities to improve performance of the computer system. TheL2 cache 208 can be organized as a 128 kbyte direct mapped cache or 256kbyte two-way set associative cache when used with 486 family processorand as a 256 or 512 kbyte direct mapped or two-way set associative cachewhen used with Pentium family processors. A cache and memory controller(CMC) and PCI bridge chip 210, is connected to the control portion 206and to the address portion 204. The CMC 210 is connected to the L2 cache208 as it incorporates the cache controller and therefore controls theoperations of the cache memory devices in the L2 cache 208. The CMC 210is also connected to control a series of address and data buffers 212.The data buffers 212 are utilized to handle memory data to a main memoryarray 214. The data buffers 212 are connected to the processor dataportion 202 and receive control signals from the CMC 210. The databuffers 212 provide a memory address bus 216 and a memory data bus 218to the memory array 214. A memory control signal bus 220 and memoryaddress bus 216 is provided from the CMC 210. Clock distribution andgeneration circuitry 222 is associated with the processor card P and isconnected to the CMC 210. A processor connector 224, such as a cardedge, is provided to be mateably received by the processor connector114. The processor connector 224 is connected to the CMC 210, the databuffers 212 and the clock distribution circuitry 222 to provide clocksto the computer system and to provide a PCI interface to allow theprocessor 200 to access the PCI and EISA buses P and E and to allow PCIand EISA bus masters to access the main memory array 214.

Referring now to FIG. 3, an exemplary video graphics adapter is shown. Avideo graphics controller 300 is connected to a PCI graphics connector316, which is mateably received in the graphics connector 116. A ROM 302is connected to the graphics connector 316 and receives control signalsfrom the video graphics controller 300. Video memory 304 is used tostore the graphics data and is connected to the video graphicscontroller 300 and a digital/analog converter (DAC) 306. The videographics controller 300 controls the operation of the video memory 304,allowing data to be written and retrieved as required. A video connector308 is connected to the DAC 306. A monitor (not shown) is connected tothe video connector 308.

It is noted that the above description of a computer system is providedfor completeness and numerous variations could be developed as apparentto those skilled in the art.

Referring now to FIG. 4, a block diagram of the memory controller 210 isshown. There are three main control blocks in the memory controller 210and three primary address transfer queues. The three primary controlblocks are the processor control or PCON block 400, the PCI control orICON block 402, and the memory control or MCON block 404. The PCON block400 provides the interface to the processor bus PB, particularly theprocessor address bus 204 and the processor control bus 206.Additionally, the PCON block 400 is connected to the L2 data cache 208and provides the necessary control signals. The L2 cache controller iscontained in the PCON block 400. In addition, the PCON 400 providessignals to control the data buffers 212. The ICON block 402 providesdata buffer control signals to the data buffer 212 and in additioninterfaces to the PCI bus 98, particularly, the control and addressportions. The MCON block 404 is the memory device control portion and isconnected to the memory address bus 216 and the memory control bus 220,and additionally provides signals to control the data buffers 212. Eachof the particular control blocks 400, 402 and 404 control differentportions of the data buffers 212 as will be illustrated.

The memory controller 210 and data buffer 212 are effectively organizedas a triangle or delta so that data transfer can occur between theprocessor bus PB and the PCI bus 98, between the processor bus PB andthe memory bus, and between the PCI bus 98 and the memory bus. To thisend the PCON block 400 is connected directly to the ICON block 402 toprovide read addresses for read operations by the processor 200 to thePCI bus 98. The write addresses are provided from the PCON 400 to a P2Iqueue 406. Preferably, the P2I queue is four operations deep, so thatfour write operations can be posted to the queue and thus be pending atone time. It is noted that in the case of a 64 bit Pentiummicroprocessor 200, this would translate to 8, 32 bit PCI bus 98operations. The output of the P2I queue 406 is provided to the ICONblock 402 to provide an address stream. In similar fashion, the readaddress information and address bank information is provided from thePCON block 400 to the MCON block 404 for processor reads from the mainmemory 214. The write addresses and bank information are provided fromthe PCON block 400 to a P2M queue 408, which is again also preferablyfour quadword addresses deep, so that four write operations from theprocessor 200 can be posted to the memory array 214 if a Pentiumprocessor and eight operations if a 486-based processor. The output ofthe P2M queue 408 is provided to the MCON block 404.

The ICON block 402 is connected to the MCON block 404 to provide readaddresses and memory bank information for read operations from the PCIbus 98 to the memory 214. In addition, the write address information andbank information is provided from the ICON block 402 to an I2M queue410. Preferably, the I2M queue 410 is capable of posting eight quadwordaddresses to provide relatively deep posting from the PCI bus 98 to thememory 214. The output of the I2M queue 410 is connected to the MCONblock 404. An SMAP or system map block 412 is connected to the controlblock 400, 402 and 404 to provide various signals as necessary.

In addition, a signal referred to as M2IABORT is provided from the ICONblock 402 to the MCON block 404 to allow the MCON block 404 to determinewhen a Memory Read Multiple operation has been aborted by the PCI busmaster, as well as for other PCI abort operations. For this discussion,only the Memory Read Multiple case is of interest and the other caseswill not be discussed. The MCON block 404 provides a snoop request orSNPREQ signal, the M2I read address that is the address for memory toPCI read operations and the I2M write address to the PCON block 400.This allows the PCON block 400 to perform snoop operations with the L2cache controller and to provide the operation to the processor 200 sothat the L1 cache controller inside the processor 200 can also perform asnoop operation. Snooping of read addresses as well as write addressesis necessary because the L2 cache controller, and the L1 cachecontroller in the processor 200 in certain cases, are preferablyorganized as writeback cache controllers, and therefore, snoopoperations must occur on reads to maintain memory coherency. The PCONblock 400 provides the SNPDONE and SNPHITM or snoop done and snoop hitto modified signals to the MCON block 404 to allow the MCON block 404 toproceed with the read or write operations or retry a read operation ifappropriate.

Referring now to FIG. 5, more details of the data buffers 212 are shown.It can be noted that the data buffers 212 also contain queues 411, 413and 414 similar to those in the memory controller 210 so that addressesare tracked in the memory controller 210 while the data is maintainedand transferred through the data buffers 212. The memory controller 210is organized to control the data buffers 212 such that the particularcontrol blocks 400, 402 and 404 control their respective portions of thedata buffers 212. For example, the PCON block 400 controls the latchingof data from the processor data bus into the various registers and theoutput enable or driving of the data from the memory onto the processordata bus 202. Similarly, the ICON block 402 handles latching of datainto the various registers from the PCI bus 98 and the output enables ofthe data to the PCI bus 98. The MCON block 404 handles the operation ofshifting data through the various queues 411, 413 and 414, driving datafrom the queues 411, 413, 414 to the actual memory devices and latchingdata as required from the memory devices into either the I2M queue 414or to registers as provided to the processor data bus 202 and the PCIbus 98. It is noted that the processor to memory and processor to PCIqueues 411 and 413 are unidirectional in the data buffers 212 but thePCI to memory queue 414 is operated bidirectionally, that it is used forboth write data and the read ahead data. Operation will be cleareraccording to the description below.

One of the features of the memory controller 210 of the preferredembodiment is that it allows great flexibility in the use of variousspeed microprocessors and various speed DRAM devices in the memory array214. Indeed, it allows the memory devices to vary from bank to bank, themain memory array 214 preferably being formed of eight banks or modules.The memory controller 210 of the preferred embodiment allows threedifferent memory speeds to be utilized, with each bank being designatedas one of the particular speeds. Each particular memory speed then has aseries of parameters relating to timing values as commonly utilized inDRAM specifications. For each particular memory speed, a value toindicate the row address hold time from RAS* is provided, the columnaddress set up times to the CAS* strobe for both reads and writes areprovided, the row address set up time to the RAS* signal is provided, asis the memory data set up time to the CAS* strobe and the pulse width ofthe CAS* strobe. In addition, the RAS precharge time is provided foreach memory speed. An indication of when the cycle ready signal isprovided to the processor 200 or PCI bus 98 is also indicated as atiming parameter. Preferably, these timing parameters utilize 10 bits ofmemory in a 16 bit register, the organization shown in Table 1.

                  TABLE I                                                         ______________________________________                                        Memory Timing Control Register                                                ______________________________________                                        Bit 15       Reserved                                                         Bits 14:13!  RAS precharge                                                                 00 = 2 clocks                                                                 01 = 3 clocks                                                                 10 = 4 clocks                                                                 11 = 5 clocks                                                    Bits 12:11!  MRDY read state                                                               00 = Assert MRDY in the C1 state                                              01 = Assert MRDY in the C2 state                                              10 = Assert MRDY in the C3 state                                              11 = reserved                                                    Bits 10:9!   reserved                                                         Bit 8        CAS pulsewidth                                                                0 = 2 clocks                                                                  1 = 3 clocks                                                     Bits 7:5!    Reserved                                                         Bit 4        MD setup to CAS                                                               0 = 1 clock setup time                                                        1 = 2 clocks setup time                                          Bit 3        Row Address Setup to RAS                                                      0 = 1 clock setup time                                                        1 = 2 clocks setup time                                          Bit 2        Column Address Setup to CAS, writes                                           0 = 1 clocks setup time                                                       1 = 2 clocks setup time                                          Bit 1        Column Address Setup to CAS, reads                                            0 = 1 clocks setup time                                                       1 = 2 clocks setup time                                          Bit 0        Row Address Hold from RAS                                                     0 = 1 clock hold time                                                         1 = 2 clocks hold time                                           RAS Precharge defines the number of                                           clocks in which all RAS* signals must be                                      negated (high) before any are allowed to be                                   asserted (low) again.                                                         MRDY read state defines which CAS state                                       MRDY should be asserted for a memory data                                     read. If the CAS pulsewidth is two clocks                                     wide, C2 is skipped. The MRDY state varies                                    on clock frequency and memory speed.                                          CAS pulsewidth defines how many clocks                                        CAS* signals must be held asserted.                                           MD Setup Time to CAS defines how many                                         clocks the MD bus must be driven from the                                     data buffers 212 before CAS* is asserted for                                  a write.                                                                      Column Address Setup to CAS, writes                                           defines the number of clocks after the Column                                 Address is generated before CAS* can be                                       asserted for a write cycle.                                                   Column Address Setup to CAS, reads                                            defines the number of clocks after the Column                                 Address is generated before CAS* can be                                       asserted for a read cycle.                                                    Row Address Hold from RAS defines the                                         number of clocks after RAS* asserts before                                    the Row Address may change.                                                   ______________________________________                                    

Referring then to FIG. 6, each memory bank includes a related register500, 502 and 504 to indicate the ending address of the particular memorybank. These addresses can be written by the processor 200 afterdetermining the particular size of the memory module located in theparticular bank. The output of the bank end address registers 500, 502and 504 are then provided as one input to comparators 506, 508 and 510.A second input to the comparators 506, 508 and 510 is provided by thememory address of the particular module such as the processor bus PB orPCI bus 98 as appropriate. It is noted that only three of the eightbanks are shown for simplicity. The comparators 506, 508 and 510 havetwo outputs and a third input. One output is the less than output, sothat if the provided memory address is less than the ending address ofthe particular bank, and there is a carry in value to indicate enablingof the comparator, the less than value is true or high, indicating thatthis is the selected bank. The comparator second output is a greaterthan or equal to signal, which is used to enable the next comparator.For example, comparator 506 has its greater than or equal to outputconnected to an enable input of the comparator 508 such that if thememory address is greater than or equal to the address provided in thebank 0 end address register 500, the comparator 508 is activated. If theaddress is then less than the bank 1 ending address as stored in theregister 502, then the output of the BANK 1 signal of the comparator 508is activated. If the memory address is instead higher, then this chaincontinues to determine if the memory address is less than that containedin any of the banks. Therefore, there are eight particular bank outputs,BANK 0 to BANK 7, to indicate the presence of the memory address in theparticular memory bank.

These BANK 0 to BANK 7 signals are provided to a memory bank valueencoder 512, which provides the BANK 2:0 signals, which are stored alongwith the write address values in the I2M queue 410 or P2M queue 408, forprovision to the MCON block 404.

As noted above, each bank could be one of three particular memoryspeeds, and these indications are contained in registers 514 and 516.Only two registers are shown for simplicity. The outputs of theregisters 514 and 516 are two bits to indicate the particular speed, andthese bits are provided to the inputs of an eight input, 2 bitmultiplexor 518 whose select input is connected to the BANK 2:0 signals.The BANK 2:0 signals are provided with the particular address, be itread or write addresses for the PCON block 400 or the ICON block 402, tothe multiplexor 518 so that the output of the multiplexor 518 is theparticular two bits that indicate the bank for the particular addressfor which the MCON block 404 is about to execute the a memory operation.The output of the multiplexor 518 is connected to the select input of a3 input multiplexor 520. The multiplexor 520 is connected to theregisters 522, 524 and 526 which contain the memory speed timing valuesfor speed 0, 1 and 2, respectively. These memory timing values are thoseindicated in Table 1. The output of the multiplexor 520 is timing valuesas utilized in the state machines and logic described below for the MCONblock 404. In addition, the address values 27..3 and byte enable or BE*7..0 signals are also provided to the MCON block 404 as they have beentracked through the queues 408 or 410 or are provided directly on readoperations. Therefore, the memory bank speed timing values are directlyprovided by first decoding the particular bank, then having a look upfor the bank based on the previously defined and stored memory speedvalue of the optional memory speed settings, and then selecting theparticular memory speed values for operation for that particular memorycycle.

An example of the timing values for the preferred embodiment for threeprocessor speeds and two memory device speeds is shown in Table II.

                  TABLE II                                                        ______________________________________                                        Memory Timing Control Combinations                                                   33 MHz    50 MHz      60 MHz                                           Parameter                                                                              60 ns   70 ns   60 ns 70 ns 60 ns 70 ns                              ______________________________________                                        RAS precharge                                                                          2       2       3     3     3     4                                  MRDY asser-                                                                            C1      C1      C3    C3    C2    C3                                 tion state                                                                    CAS pulse-                                                                             2       2       2     2     2     3                                  width                                                                         MD Setup 1       1       1     1     2     2                                  Write Column                                                                           1       1       1     1     1     1                                  Addr Setup                                                                    Read Column                                                                            1       1       1     1     1     1                                  Addr Setup                                                                    RA hold  1       1       1     2     2     1                                  Memory Tim-                                                                            00.sub.-- 00                                                                          00.sub.-- 00                                                                          30.sub.-- 00                                                                        30.sub.-- 01                                                                        20.sub.-- 11                                                                        51.sub.-- 10                       ing Control                                                                   Register (hex                                                                 value)                                                                        ______________________________________                                    

The DRAM device speeds can be determined by reading encoded bitsconventionally provided on SIMMs and checking the size. These twoparameters are then used in a lookup table to determine SIMM speed. Thistechnique is described more fully in Ser. No. 08/034,105, filed Mar. 22,1993, which is hereby incorporated by reference. With the DRAM devicespeed then known for each bank and microprocessor speed known by otherencoded bits, these two parameters are then used in a second lookuptable to obtain the binary value to be placed in the timing valueregister for that continuation. The computer system designer knows thevarious propagation delays of the various paths and can then simply usethe clock speed and memory timing parameters provided by the memorydevice manufacturer to determine the timing values for each combination.

Proceeding then to FIG. 7, the use of the addresses, byte enables andtiming values are shown as being provided to a portion of the MCON block404. The processor write request, processor read request, refresherquest, PCI read request and PCI write request signals are inputs to anarbiter 550 along with signals referred to as I2CAMHIT, I2MQ₋₋ EMPTY andP2MQ₋₋ EMPTY. The various request signals are an indication thatoperations are pending for the memory controller to operate on from theindicated sources of the indicated type. Development of these signals isnot fully described but can be readily developed by one skilled in theart. The outputs of the arbiter 550 are the P2M, M2P, I2M and M2Isignals to indicate which source and direction of operation hasoccurred, that is P2M for a processor to memory write, M2P for aprocessor to memory read, I2M for a PCI to memory write operation andM2I for a PCI read operations. These four signals are provided as theselect signals to a four input multiplexor 552 which receives theparticular read or write addresses, the byte enables, the memory timingvalues and the bank indications for the particular source. The P2M queue408 provides write addresses, byte enables, memory timings and bankinformation from the queue 408 for processor to memory write operations,while the read addresses, byte enables, memory timings and bankinformation are provided directly for processor reads in the M2P case.Similarly, the I2M queue 410 provides write addresses, byte enables,memory timings and bank information from the I2M queue 410, while theread addressing information is provided directly from the ICON block 402to the MCON block 404. The output of the multiplexor 552 is theparticular address value, the byte enables, the bank value to indicatewhich particular bank and the memory timing values for the particularmemory operation to be performed by the memory controller 210 to accessthe necessary memory location for either a read or a write asappropriate. It is also noted that a page hit detector 554 is connectedto the output of the multiplexor 552 to provide a PAGEHIT signal toallow the memory controller 210 to determine whether a page hit hasoccurred, so that it can operate the DRAM devices in the desired pagemode operation for best performance.

Proceeding then to FIG. 8, blocks representing various state machinesand other logic utilized in the MCON block 404 are shown. A number ofstate machines provide the actual information and control functions andare interlocked. The primary state machine is the MEMSM or memory statemachine 600. The memory state machine 600 receives the memory timingvalues from the multiplexor 552 and the M2IABORT signal from the ICONblock 402. A precharge state machine 602 is provided to provide a signalreferred to as RASPCHG or RAS precharge to indicate that the prechargetime for the particular memory bank is completed if page mode operationis not occurring. The RASPCHG signal is provided to the memory statemachine 600. Additionally, RASUP or RAS up logic 604 provides a RAS₋₋ UPsignal which is utilized by the memory state machine 600 to determinewhether to leave the RAS* or row address strobe signal high or low whenno memory cycles are pending, thus providing a prediction whether thenext operation will be a page hit or page miss to thereby improveperformance. Detailed operation of this logic will be described below.

A refresh state machine 606 is provided to control refresh operationsbut will not be described in detail in this description because itsoperation is relatively conventional and is omitted for clarity. Amemory address state machine 608 receives the timing values from themultiplexor 552, as well as the basic clock signal of the system andprovides interlock signals to the memory state machine 600, the MSELRAor memory select row address signal and the MWE* or memory write enablesignal. The MSELRA signal is provided to the select input of a 2:1multiplexor 610 which receives at its inputs the memory addressesappropriately connected to provide row and column addresses based on theselection input. A burst address block 612 is provided and connected tothe column addresses to simplify burst operation. The outputs of themultiplexor 610 are the memory addresses provided to the memory array214 over the memory address bus 216 for the particular operation. TheMWE* signal is similarly the memory write enable signal as provided tothe main memory array 214. A memory data control state machine 614 isprovided. It receives certain of the memory timing values and providesinterlock signals to the memory state machine 600 and controls theoperation of pertinent portions of the data buffers 212.

The memory state machine 600 provides the MRAS or master RAS and MCAS ormaster CAS signals. The MRAS signal is combined with the bank valuesignals for the particular memory operation to provide the RAS* 7:0signals which are provided to the particular banks. If the bank valueindicates that this is the particular bank value for the operation, thenthe RAS logic 616 directs the MRAS signal to that particular bank in aninverted form. If the bank value is not the one particularly beingaddressed, then the RAS* signal for that particular bank is held high toallow the memory devices to be fully precharged. The MCAS signal isprovided to CAS logic 618 which also receives the BE* 7:0 signals forthe preferred 64 bit width. These signals provide the eight byte lanesof information that are encoded with the MCAS signal to produce the CAS*7:0 signals which are used to enable the particular memory devices ofthe bank indicated by the RAS* 7..0 signals. If a particular byte enableis not activated, then its related CAS* signal is not activated, andtherefore the memory device is not selected as it does not receive acolumn address. This allows the use of a single MWE* signal, with theCAS* signal providing the byte lane encoding.

The queues 406, 408 and 410 are organized as a series of registers ofthe appropriate width. Particular registers in each queue are selectedbased on three particular counter values associated with each queue. Thethree counters are the read location counter, the write location counterand the data contents counter. The read location counter is utilized todetermine the particular location of the four or eight registers fromwhich the read data for the next cycle is to be obtained and provided tothe particular block. The write location counter is utilized to indicatethe particular register into which data is to be written in the nextoperation. The read and write location counters increment on each readand write operation, respectively, and thus operate in a circularfashion. The data contents counter is utilized to indicate whether thereis actually data in the queue. The data contents counter is an up/downcounter. The data contents counter counts up on a write operation to thequeue and decrements on read operations. If the data contents counterindicates that the queue is full by being at a maximum value, then datais not written into the queue until data has been removed from thequeue. These counters are conventional logic and are not shown forsimplicity and clarity. I2MQ₋₋ EMPTY and P2MQ₋₋ EMPTY signals thuscorrespond to the data contents counters being at a zero value.

Because of the length of the I2M queue 410, it is organized as a contentaddressable memory or CAM. I2M queue registers 640 and 642 are shown inFIG. 9, with many omitted for simplicity. The output of the particularregister 640, 642 is not only provided to the MCON block 404 via amultiplexer but is also provided to an associated comparator 644 and646. The second input of the comparator 644 and 646 receives theprocessor address being provided on the processor address bus 204. Thecomparators 644 and 646 have equal outputs so that if the address beingprovided directly on the processor address bus 204 is equal to one ofthe addresses contained in the I2M queue 410, then the particularcomparator provides its equal signal true. The equal signal output goesto one input of an AND gate 648 and 650. The second input to the ANDgate 640 and 650 is provided by a valid bit register 652 and 654. Ifdata is not contained in the particular register 640, 642 in the I2Mqueue 410, then the valid bit register is set to 0, whereas if data ispresent, then the bit is set to 1. These valid bit registers are setwhen data is written into the I2M queue 410 and cleared when data isread out of the queue 410. In this manner, if the register is valid andthe comparator determines that there is an equality, the output of theparticular AND gate is set to a one value to indicate that a hit hasoccurred. The outputs of the AND gates 648 to 650 are provided to aneight input OR gate 654 whose output is the I2MCAMHIT signal, which isprovided to indicate that the particular read address being requested bythe processor is actually present in the I2M queue 410 as a writeoperation from the PCI bus 98, and therefore the memory read requestfrom the processor 200 cannot occur until the particular location hasbeen flushed out of the I2M queue 410. Operation of this delay is seenin the arbiter 550 as described below.

In the preferred embodiment, there are five possible requests for themain memory 214: a processor-to-memory write (P2M), amemory-to-processor read (P), a PCI-to-memory write (I2M), amemory-to-PCI read (M2I), and refresh. A P2M write refers to a write tothe memory 214 and a M2P read refers to a read of the memory 214, bothinitiated by the microprocessor 200. An I2M write refers to a write tothe memory 214 and a M2I read refers to a read of the memory 214, bothinitiated by a PCI bus master. All memory requests from EISA bus mastersare passed through the PCI-EISA bridge 130, which includes the necessarylogic to be a bus master on the PCI bus 98. Thus, any EISA originatedmemory request is effectively a memory request asserted by the PCI-EISAbridge 130.

Generally, the priority of the memory requests are as follows, with someexceptions: (1) second refresh request; (2) P2M write request; (3) M2Pread request; (4) I2M write request; (5) M2I read request; and (6) firstrefresh request. The second refresh request indicates that two refreshesare outstanding. When that occurs, the memory controller 234 gives bothoutstanding refresh requests the highest priority, executing bothrefresh cycles. The P2M write request is always higher in priority thanother memory requests except the second refresh. However, if the I2Mqueue 410 is full or the I2M queue 410 is not empty and a PCI bus 98read is outstanding, a signal M₋₋ P2M₋₋ NOPOST is asserted to preventfurther queuing of P2M write requests until a PCI write operation iscompleted. This allows the P2M queue to clear out, thereby allowingrequests from the PCI bus 98 to be serviced. However, assertion of thesignal M₋₋ P2M₋₋ NOPOST does not prevent writeback cycles from beingqueued, as the writeback may be needed by the PCI memory request.

The M2P read request is always lower in priority than the P2M writerequest, but it is usually higher in priority than I2M write and M2Iread requests. The operation can only occur when the P2M queue 408 isempty. However, an unlocked M2P read request is forced lower in prioritythan an I2M write request if the M2P read is to the same address as anI2M write pending in the I2M queue. When this occurs, the M2P requestremains lower in priority than I2M requests until the I2M write requesthaving the matching address is written to the main memory 214. A M2Pread request is also forced lower in priority than an I2M write requestif the I2M queue is full. Additionally, if an M2I read request isasserted while an I2M write request is pending, the I2M write request isforced higher in priority than the M2P read request to allow the I2Mqueue to clear, thereby allowing the M2I request to proceed. Further, anM2I read request is forced higher in priority than the M2P read requestif the M2I read has been waiting for the M2P request to negate for morethan one arbitration cycle.

The I2M write request is always lower in priority than the secondrefresh request, the P2M write request, and it is generally lower inpriority than the M2P read request with the exceptions noted above. TheI2M write operation can only occur when the P2M queue 408 is empty. TheI2M write request is always higher in priority than the M2I readrequest. The I2M write request is held off if the processor isperforming a locked access of the main memory 214. Thus, for a lockedprocessor cycle, the exceptions discussed above do not apply to overridethe higher priority of M2P read requests over I2M or M2I requests.

A locked or atomic access of the main memory 214 is indicated by asignal LOCK* driven by the microprocessor 200. A locked cycle allows themicroprocessor 200 to read an address location in the main memory 214and be assured that the accessed location is not changed by another busmaster before the microprocessor 200 writes back to the same memorylocation. These type cycles are referred to as read modify write cycles.Locked cycles are also generated during other bus transfers, such asduring execution of the XCHG (exchange) instruction when one of itsoperands is memory-based, when updating a segment or page table entry,and when executing interrupt acknowledge cycles.

The M2I read request is always lower in priority than the second refreshrequest, the P2M write request, and the I2M write request. However, itis higher in priority than the unlocked M2P read request in the instancenoted above. The M2I read operation can only occur when the P2M queue408 is empty.

Finally, the first refresh request is always lower in priority than anyof the other requests. However, as noted above, when the second refreshrequest is asserted, both the first and second refresh requests areexecuted regardless of whether other requests are pending.

Further details of this arbitration are provided in Ser. No. 08/324,011,issued Jun. 4, 1996 as U.S. Pat. No. 5,524,235, entitled System forArbitrating Access to Memory filed concurrently herewith, which ishereby incorporated by reference.

Referring now to FIG. 10 the memory state machine 600 is shown indetail. Indications in italics next to a particular state bubbleindicate the value of various output signals, developed by the statemachine. In this case the MRAS and MCAS signals. The phrases adjacent toan arrow going from one bubble to an other are the logical conditions onwhich that path is taken. If a path does not have an adjacent logicalcondition, that path is taken in all other cases. Transitions are madeon the rising edge of the processor clock signal. Upon reset of thecomputer, operation of the state machine 600 transfers to the RI or RASidle state. In this state the MRAS signal is not asserted if the nextstate is not state R1. If the next state is R1, then the MRAS signal isset to the value 1 or asserted so that the row address strobe signal isprovided. The MCAS signal is negated to a zero level. Control proceedsfrom state RI to state R1 if the M2P condition is true so that aprocessor read operation is occurring, if a PCI read operation isoccurring (M2I) which has not been aborted (|M2IABORT) and is not beingcleared (|CLRI2M), or if a write operation from the processor or the PCIbus is occurring; the row address ready signal (RADRRDY) is true, whichindicates that the row address set up time to the RAS signal has beenmet as indicated by the memory address state machine 608, or, if in aprocessor read case there is no wait signal indicated as would beappropriate if an aborted memory to PCI read ahead is completing(WAITRADRRDY) and the row address set up time two (RADSETUP2) signal isnot activated which indicates that relatively fast memory is present; arefresh cycle is not occurring (RFSH₁₃ ON); the current read cycle tothe PCI bus 98 is not being aborted (M2IABORTCUR); and the prechargeperiod has completed for as indicated by the RASPCHG signal. TheM2IABORT signal is provided by the ICON block 402 as described above.The M2IABORTCUR signal indicates that the cycle currently beingperformed by the MCON block 404 is to be aborted or terminated as soonas possible. The M2IABORTNEXT signal indicates that the next M2I cyclewhich is pending is to be aborted, which occurs by simply skipping thecycle. The development of the CLRI2M signal is detailed below, butbriefly indicates that a snoop read hit to a modified location hasoccurred, so that the I2M queue 414 must be flushed and the read cyclereexecuted. Therefore, if a memory cycle is indicated, the row addressset up time has been met, and the device is precharged, control proceedsin state RI to state Ri. In all other cases control remains at state RI.

In state R1 the MRAS signal is set to 1 or high and true, and the MCASsignal is set low to indicate that this is a RAS only portion of thecycle. Control proceeds from state R1 to state R2 if the RADHLD2 signalis true, which indicates that two clock periods of hold time arerequired and therefore the intermediate R2 state is required. If theRADHLD2 signal is not true, then only 1 clock of hold time is requiredfrom the activation of the RAS* signal and control proceeds directlyfrom state R1 to state R3. In state R2, the MRAS signal is asserted andthe MCAS signal is negated. Control proceeds from state R2 to state R3on the next rising edge of the processor clock signal.

In state R3, the MRAS signal is true or high, while the MCAS signal isset high if the next state is to be state C1 and otherwise the MCASsignal is set low. Control proceeds from state R3 to state C1, the firstof the column states, if the column address ready (CADRRDY) signal istrue as provided by the memory address state machine 608 to indicatethat the particular column address set up time, be it read or write, hasbeen satisfied and either a PCI read operation is occurring or a memoryto processor bus read operation is occurring, and the memory operationis not stalled waiting for read data; or a write operation from theprocessor to memory or the PCI bus to memory is occurring with the databeing valid as indicated by the MDVALID signal which is provided by thememory data state machine 614. Therefore, if the column address set uptime has been met and indication of write operation data being properlyvalid for write cases is provided, control proceeds from state R3 tostate C1. Otherwise, control remains in state R3 waiting for the timingconditions to be satisfied.

In state C1, both the MRAS and MCAS signals are high to indicate thatthe CAS portion of the cycle is occurring. Control proceeds from stateC1 to state C2 if the CASPW3 or CAS pulse width 3 clock signal is true.If the CASPW3 signal is not true, control proceeds directly from stateC1 to state C3, with control always proceeding from state C2 to state C3on the next rising edge of the processor clock signal. In state C2, theMRAS and MCAS signals are both also true. Thus, state C2 is skipped ifthe CAS pulse width can be narrower, as would be true for faster memoryor slower processors with an additional clock period provided whennecessary.

In state C3 the MCAS signal is negated to a 0 level to indicatecompletion of the particular memory cycle and the MRAS signal is set to0 if the next state is the RI state or if the particular cycle isaborting. Otherwise, the MRAS signal remains in a high or asserted stateto allow operation in page mode if appropriate. Control proceeds fromstate C3 to state RI if a burst operation is not occurring, which is apreferable operation for 486 and Pentium processors, or the currentmemory to PCI read cycle is being aborted as explained below; the nextmemory to PCI read operation is not aborted; there is no indication thatthe M2I cycle is to be aborted as indicated by the M2IABORT signal orthe CLRI2M signal; and a non-refresh cycle is occurring (M2P+ADDRRDY)and this is not a page hit; a refresh cycle is occurring or there is nocycle pending and the RAS₋₋ UP signal is true, indicating that the RASUPblock 604 has indicated that the prediction is for the next cycle to bea page miss. The RAS₋₋ UP signal is generated as follows:

    ______________________________________                                        RAS.sub.-- UP =                                                                        |((|WRITEBACK && P2M)                                                                          (|MDATARD && M2P)                                            (|Q.sub.-- I2MQEMPTY && I2M)                                                                   (|M2IABORTCUR &&                                             M2IMULREAD &&    HIT2MOD)                                                     |M2IABORTNEXT)                                                       ______________________________________                                    

Therefore, the RAS* signal is kept low or in page mode followingprocessor to memory write operations which were not writebacks from theL1 or L2 caches, processor code read operations, PCI bus writeoperations with pending write operations, Memory Read Multipleoperations from the PCI bus 98 which are not being aborted, or hits to amodified address, indicating that a writeback operation has justoccurred. In all other cases the RAS₋₋ UP signal is true, such that itis considered better to initiate a full new page access to allowprecharging to occur during the idle period.

If the conditions for transfer from state C3 to RI are not true, controlproceeds from state C3 to state RN or the RAS low idle state. In thisstate the MRAS signal is low if the next state is state RI otherwise theMRAS signal is asserted. The MCAS signal is asserted if the next stateis C1, otherwise the MCAS signal remains in low state. Control proceedsfrom the RN to C1 state for burst operations in general if the columnaddress set up time has been met and there are no abort operationsoccurring. Control proceeds from the RN state to the RI state underconditions which generally indicate that the cycle is being aborted or aprocessor non-burst read which is a page miss is next to occur. In allother cases, control remains at state RN, idling with the RAS* signallow until a cycle needs to be run or an abort or page miss occurs.Therefore, it can be seen that the memory state machine 600 drives theMRAS and MCAS signals based on the timing values of the proper bank,these timing values being variable as indicated above.

A memory address state machine 608 is shown in FIG. 11. The statemachine 608 starts at state RAI upon reset. In this state, the RADRRDYsignal and CADRRDY signals are set low to indicate that the row andcolumn addresses are not ready, and the MSELRA signal is set high toinitially select the row address to be provided to the memory devices.The MWE* signal is set to 1 or negated level on reset and after entryinto the RAI state and is set equal to the CH1 state value the firststate after entry from the CH1 state as described below. Controlproceeds from the RAI state to the RH1 state if an active cycle ispending from the PCI bus 98 or processor 200 and the next cycle in theread ahead operation is not to be aborted. The current M2I read cycle isaborted under certain conditions, such as receipt of M2IABORT signal ora writeback is occurring, while the next M2I read cycle is aborted whenthe M2INA signal has been received but the cycle not started when theM2IABORT signal is received. In all other cases control loops at stateRAI.

In state RH1 the CADRRDY signal is set low and the RADRRDY signal is setto the value of M2P or memory to processor read upon entry from the RAIstate, is equal to 0 on entry from the CH2 state as defined below andafter entry into the RH1 state is set to a 1 level. The MSELRA and MWE*signals are set to high levels so that the row address is selected and awrite operation is not indicated. Control proceeds from the RH1 to theRH2 state if the MRAS signal is true from the memory state machine 600and the RADHLD2 signal or row address hold time signal is set toindicate slower memory devices. If the MRAS signal is true and theRADHLD2 signal is not set, control proceeds from the RH1 state directlyto state CH1, which is also where control proceeds on the next clocksignal in the RH2 state. In all other cases, operation loops at the RH1state. In the RH2 state, the RADRRDY signals and CADRRDY signal are setlow, and the MSELRA and MWE* signals are set at high state. Controlproceeds from the RH2 to the CH1 state.

In the CH1 state, the RADRRDY signal is low to indicate that the rowaddress set up time has not been met. The CADRRDY signal is set to ahigh or true value after initial entry into the state. On entry theCADRRDY signal is set high if short CAS address setup times were set(|CADSETUP2) for the read or write as appropriate, and otherwise is setlow. The MSELRA signal is set to 0 to indicate that the column addressesare selected and the MWE* signal is low if a write operation isoccurring.

Control proceeds from the CH1 state to the RAI state if the memory statemachine 600 is in state RN and either the cycle is aborting or aprocessor read page miss is pending. Control proceeds from the state CH1to state CH2 if the memory state machine is not in state RN or the cycleis not aborting completely or the processor read cycle is a page hit. Inthe CH2 state the RADRRDY and CADRRDY signals are set low to indicatethat no addresses are ready. The MSELRA signal is set low and the MWE*is set low to indicate the write operation is occurring and the columnaddresses are provided. Control proceeds from the CH2 state to the CH1state for ongoing burst cycles when the memory state machine is in stateC3 or for pending processor read page hit operation. Control proceedsfrom the CH2 state to the RAI state if the cycle is aborting or if anidle condition is present and the RAS* signal is to be set high. Controlproceeds from the CH2 to the RH1 state if a processor read page missoperation is to occur and the memory state machine 600 is at the end ofa cycle.

Therefore the memory address state machine 600 properly provides theMWE* signal for the DRAM devices and controls the address multiplexor610. In addition, the RADRRDY and CADRRDY or row and column addressready signals are provided to the memory state machine 600 based uponthe column and row address set up times as provided in the memory timingregisters.

The MD or memory data state machine 614 is illustrated in FIG. 12. Thestate machine 614 starts at the IDLE state upon reset. Control proceedsfrom the IDLE state to a RD state for processor or PCI bus 98 readoperations which are not being aborted and refresh is not occurring.Control proceeds to the DCD state if a processor to memory writeoperation is occurring and there is no refresh or if a PCI bus to memorywrite operation is occurring and again there is no refresh. Controlotherwise remains at the IDLE state. Control proceeds from the RD stateto the DCD state if the memory state machine is in states C3 or RN, thecycle is either being aborted or is not a burst cycle and it is a writeoperation with a 33 MHz processor. Control proceeds from the RD state tothe IDLE state if MRAS is negated and the cycle is aborting or thememory state machine is in states C3 or RN, the cycle was not a burst oris aborting, and a non-33 MHz processor write operation or PCI bus 98write cycle is pending or no cycles are active. In all other cases,control remains at state RD.

Control proceeds from the DCD state to the DH1 state if the MDSETUP2signal is not true, indicating that this is a fast memory data set upcase, and the memory state machine 600 is not in state C2 and 33 Mhzoperation is indicated for the processor. In all other cases, controlproceeds from the DCD state to the DS2 state. Control proceeds from theDS2 state to a DS3 state if the processor 200 is not operating at 33 Mhzand the MDSETUP2 signal is set to indicate slower memory data set uptimes. Control proceeds from the DS2 to DH1 states in all other cases.Control proceeds from the DS3 state to the DS1 state on the next risingedge of the clock signal.

In the DH1 state, the MDVALID signal is set to 1 or asserted to indicatethat the memory data set up time to the column address strobe has beenmet. This allows the memory controller state machine 600 to proceed tostate C1. Control proceeds from the DH1 state to the DCD state iffurther write operations are pending and otherwise proceeds to the DH2state. In the DH2 state, the MDVALID signal is set to 0 to indicate thatthe memory is no longer valid. The value of the MDVALID signal is notchanged in any states other than the DH1 and DH2 states, except it iscleared on reset. Control proceeds from the DH2 state to the IDLE stateif no write operations are pending from the PCI bus 98 or from theprocessor 200. In all other cases, control proceeds from the DH2 stateto the DCD state. In this manner, the memory data state machine 614provides the MDVALID signal to the memory state machine 600 when thememory data is properly set up.

The precharge state machine 602 is shown in FIG. 13. Upon reset, thestate machine 602 operation commences at state A. Control remains instate A when the MRAS signal is not asserted. When the MRAS signal isasserted, control proceeds to one of states B, C, D, or E, dependentupon the number of clocks defined for the precharge time. Controlproceeds to state B for five clocks, to state C for four clocks, tostate D for three clocks and to state E for two clocks. Control proceedsfrom states B to C to D to E, sequentially, when the MRAS signal is notasserted. Otherwise, control remains in each particular state. Controlthen proceeds from state E back to state A when the MRAS signal isdeasserted. Therefore the precharge state machine 602 leaves state Aupon the commencement of a particular memory operation and then does notbegin traversing the remaining states until the MRAS signal has beennegated, so that a precharge period has started. It is noted that thisresults in the proper precharge time for any given bank, even if banksare being switched in sequential memory operations if refresh operationsare ignored. This occurs because, as noted above, when a particular bankis not selected the RAS* signal for that bank is set high so that it isin a precharge period. Thus if the bank is not selected, it has beenthrough at least one full memory cycle of precharge, which is sufficientto provide the required recharge in all cases. Thus, the precharge timeis set for that of the particular memory bank on which the current cycleis operating, so that if back to back cycles occur on that particularmemory bank, the memory devices are properly precharged.

However, refresh operations somewhat degrade this operation as it is notknown in the preferred embodiment which bank will be requested after arefresh cycle, so to simplify the design of the preferred embodiment,the precharge value for all of the DRAM types is set to the worst caseby the initialization or POST software. This reduces performance incertain cases but simplifies the design. A slightly more complicateddesign would use the worst case value, preferably provided to apredetermined register for only the first precharge operation after arefresh cycle and thereafter operation would revert to the optimaltiming for each bank.

Write addresses and data traverse through the I2M queues 410 and 414based on having posted data and the arbiter 550 providing access to thequeues 410 and 414 by the memory system. The write addresses areprovided to the PCON block 400 when placed in the I2M queue 410, toallow the PCON block 400 to control snooping of the address by the L1and L2 cache controllers. Read operations occur in a different fashionthan write operations. Read addresses are provided to the MCON block 404along with an indication that a read request has issued. The readaddresses are provided based on a return of an L2 cache line, which is 432 bit words for 486 microprocessors and 8 32 bit words for Pentiumprocessors. Therefore, when an M2I read is requested, four or eight 32bit words are provided, with the ICON block 402 properly providing thedata from the read operation to the PCI bus 98 as required. The readrequest indications are provided by a memory to PCI next address statemachine described below. The read request is also provided to thearbiter 550 for arbitration and to the PCON block 400 to allow L1 and L2cache snooping with a writeback in case of a hit to a modified line.When the arbiter 550 provides the approval to proceed with the PCI bus98 read, the MCON block 404 then proceeds to process the information. Itis noted that this will have occurred only after the I2M queues 410 and414 are emptied of any pending write operations so that the I2M dataqueue 414 provided in the data buffer 212 can be utilized to do a blockread operation of the lengths indicated. Therefore, when the I2M queues410 and 414 are emptied, the read operation commences and is controlledby an M2I state machine as shown in FIG. 14. The state machine commencesat state A upon reset. Control then proceeds to state B if the I2M queue414 is empty, a read is requested and the L2 cache 208 is not beingflushed. This would be a normal read condition. If instead, the L2 cache208 is being flushed, control then proceeds from state A to state F,assuming that the I2M queue 414 is empty and the read request ispresent. Otherwise control remains at state A waiting for the readoperation to be initialized.

From state B, control proceeds to one of four different states. Controlreturns to state A if the M2IABORT signal has been received, indicatingthat the PCI bus master has aborted the read operation, or if theIREQACK signal is true, which indicates that the next operation asdetermined by the arbiter 550 will be a PCI read, and the SNPDONE signalis true indicating that the PCON block 400 has completed snooping theread operation address to the level 2 cache and to the level 1 cache inthe processor 200. Control proceeds from state B to state C if theM2IABORT signal is not asserted, the next cycle is a PCI read asindicated by the IREQACK signal being asserted, there has not been asnoop hit to a modified (SNPHITM) location as indicated by the PCONblock 400 and the snoop operation has not been completed, as alsoindicated by the PCON block 400. This will be the case where a readoperation has been requested and has been arbitrated but may have to beaborted because the snoop operation has not been performed and a hit toa modified location may yet occur. Control proceeds from state B tostate D, if the operation is not being aborted and a PCI read has notbeen arbitrated, but there has been a snoop hit to a modified locationwith the writeback operation not yet completed. Control proceeds fromstate B to state E if the cycle is not aborted, has been arbitrated asindicated by the IREQACK signal and there has also been a snoop hit to amodified location, so that a writeback operation from the appropriatecache controller will be occurring. In all other cases control remainsat state B.

Control proceeds from state C back to state A if the cycle is beingaborted or if the snoop is completed without being a hit to a modifiedlocation. Control proceeds from state C to state E if the cycle is notaborted and there has been a snoop hit to a modified location.Otherwise, control remains at state C until the snoop operation iscompleted, the cycle is aborted or there is a snoop hit to modified.Control proceeds from state D back to state A if the operation isaborted or upon indication that the PCI cycle is next for operation andthe snoop has completed. This would occur after the writeback has beencompleted and then the PCI operation is next to occur, as the write backwill have superseded the PCI operation. Control proceeds from state D tostate E if it is not being aborted, the PCI request is next and thesnoop has not been fully completed. Otherwise control remains at stateD.

In state E, the CLRI2M signal is set to indicate to other portions ofthe memory controller 210 that because of the writeback, the data in theI2M queue 414 must be flushed and discarded. A CLRI2MDONE signalindicates this flushing has been completed. Control proceeds from stateE to state F if the cycle is not being aborted and the signal CLRI2MDONEor clear the queue done signal is true and the snoop is not yetcompleted. Control proceeds from state E to state G if the cycle is notbeing aborted, the clearing of the queue 414 has been completed andsnoop has been performed. Control proceeds from state E to state A ifthe cycle is being aborted and in all other cases remains at state E.

Control proceeds from state F to state G if it is not being aborted andthe snoop cycle has been completed. Other control proceeds from state Fback to state A if the cycle is being aborted. Otherwise control remainsat state F. Control proceeds from state G back to state A if the cycleis aborted or if it is next in line as indicated by the IREQACK signal.

Therefore, the M2I state machine controls transfer of information fromthe memory to the PCI interface using the I2M queue 414 located in thebuffer 212. Data is not transferred until the queue 414 is cleared ofany write data and then proceeds only based on snoop information andwhen the arbiter 550 allows it to proceed.

As noted above, the design of the preferred embodiment performs readahead operations when a PCI Memory Read Multiple operation has beenreceived. The operation proceeds as follows. The ICON block 402 receivesa Memory Read Multiple cycle form the PCI bus 98 and when there is roomin the I2M queue 414 issues an M2I read cycle request, along with anaddress, to the MCON block 404. The MCON block 404 arbitrates asdescribed above and ultimately starts on the M2I read request. When ithas started the operation, it provides a next address or M2INA signal tothe ICON block 402 and provides the data to the I2M queue 414 along withthe appropriate ready signals. The ICON block 402 knows a Memory ReadMultiple command is occurring and issues another M2I read cycle requestat the next address when the M2INA signal is received and the I2M queue414 can receive another cache line of data. The ICON block 402 alsoreceives the ready signals from the MCON block 404 and provides the datato the PCI bus 98. The MCON block 404 receives the M2I read cyclerequest and executes it when the cycle wins the arbitration.Conventionally this will be before the ICON block 402 has removed all ofthe data from the I2M queue 414 for the initial read request. The MCONblock 404 then commences the read request and issues another M2INAsignal. The MCON block 404 then provides the data to the I2M queue 414.The ICON block 402 receives this M2INA signal and again checks to see ifthere is space available in the I2M queue 414 to receive another cacheline. When there is room because the ICON block 402 has providedsufficient data to the PCI bus 98, the next M2I read request is providedto the MCON block 404. This process continues until either the MemoryRead Multiple completes, a page boundary is crossed or the PCI busmaster aborts the cycle.

The abort case is the one of interest as the pending read aheadoperation is terminated as soon as possible to save retrieving theentire cache line. This can be seen in the discussions of the MEMSM 600,the MADRSM 608, the MDCDSM 614 and the M2I state machine. This quicktermination is seen in the MEMSM 600 as the return to the RI or C1states from the C3 and RN states, so that the cycles finishes as soon asthe current individual read operation is completed, thus potentiallybefore the completion of the full cache line read. Similarly the MADRSM608 returns to the RAl state if a cycle has not started or when thecolumn addresses have been provided. The MDCDSM 614 returns to the IDLEstate if no operation has been started or if the MEMSM 600 is in the C3or RN states. The M2I state machine returns to the A state whenever theM2IABORT signal is received. On detection of the abort, the ICON block402 determines the end of the read cycle and resets its pointers to thedata in the I2M queue 414 to indicate that no data is present, thuseffectively discarding the data which has been read ahead. Thus the readahead operation terminates as soon as possible after the abortindication is received, saving time by not completing the full cacheline retrieval.

The next address indication to allow the read ahead is provided by theM2INA or memory to the PCI next address state machine shown in FIG. 15.The initial read address will have been provided using the conventionalread request mechanism. An M2INA signal is provided to the MCON block404 to indicate that the next read cycle can being. This state machinebegins operation in state A upon reset and proceeds to state B if thePCI read operation is next as indicated by the IREQACK signal and is notbeing aborted. Otherwise, control remains at state A. Control proceedsfrom state B back to state A if the cycle is being aborted or if the I2Mqueue 414 is to be cleared because of a writeback or if the snoop hasbeen completed and the M2IACK signal is provided indicating that theprior read operation has been acknowledged by the MCON block 404.Otherwise control remains at state B. Control proceeds from state B tostate C if the cycle is not being aborted, it is not necessary to clearthe queue, the snoop operation has not completed and yet an M2IACKsignal has been received. Control proceeds from state C back to state Aif the cycle is aborted, the I2M queue 414 is being cleared, or thesnoop is completed and otherwise remains in state C. Thus the M2INAstate machine returns to idle upon receipt of an abort indication.

The M2INA signal is provided to the MCON block 404 to indicate that thenext address is being provided, that is, another read request can beissued to keep the I2M queue 414 filled ahead of the PCI bus 98. TheM2INA signal is provided if the cycle is not being aborted, the I2Mqueue 414 is not being cleared, the snoop of the previous read cycle hascompleted and the M2INA state machine is either in state C or in state Band the M2IACK signal has been received. This M2INA signal is anindication that the processing of the prior address is complete by theMCON block 404 and the processing of the next read address can begin.The actual incrementing of the read address value is performed in theICON block 402 using an 8 bit counter, thus limiting the total readahead length to 256 address values. When the counter reaches 255, theread ahead operation is terminated by logic not illustrated forsimplicity by causing the Memory Read Multiple to be disconnected. A newaddress must be received from the PCI bus master to continue the MemoryRead Multiple Operation.

Therefore a memory controller according to the preferred embodimentincludes many improvements. It is very flexible, allowing simple yethigh performance use with a plurality of processor and memory devicespeeds, allowing flexibility and economics of scale. It providesimproved performance by basing idle state RAS* signal level predictionson both processor and PCI bus conditions. It allows very deep writepoints from several buses and easily handles the various coherencyissues. It also provides a quick method for ending read ahead cycleswhen the basic read operation has been aborted.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

What is claimed is:
 1. A computer system comprising:a memory formed ofpage mode DRAMs; a processor performing read and write operations to thememory; an input/output bus which can receive bus masters which accessthe memory, the bus masters performing read and write operations to thememory, the input/output bus including address, data, and controlsignals, wherein the control signals specify the type of operationindependent from an address specified by the address signals; a memorycontroller for controlling operation of the DRAMs by providing row andcolumn address strobe signals, the memory controller including:statelogic determining if the memory is in an idle state after completion ofa memory operation; state logic storing a last type of operation asspecified by the control signals performed by the memory prior to saididle state; state logic determining if the last type of operationperformed was from a bus master on said input/output bus; and statelogic holding the row address strobe signal asserted during said idlestate predictive of a page hit to said page mode DRAMs after said idlestate if said last type of operation was from one of the bus masters onsaid input/output bus and was a predetermined type of operation asspecified by the control signals independent from the address specifiedby the address signals and otherwise setting the row address strobenegated during said idle state predictive of a page miss to said pagemode DRAMs after said idle state if said last type of operation was fromthe one of the bus masters on said input/output bus and was not saidpredetermined type of operation as specified by the control signalsindependent from the address specified by the address signals.
 2. Thecomputer system of claim 1, and further comprising a hard disk drive forstoring information coupled to said input/output bus.
 3. The computersystem of claim 1, and further comprising a non-volatile memory thatcontains basic input/output system (BIOS) information, the non-volatilememory coupled to the input/output bus.
 4. The computer system of claim1, wherein said input/output bus includes read operations indicatingmultiple read operations to occur and wherein said predetermined typeincludes the multiple read operation.
 5. The computer system of claim 4,wherein said memory controller further includes:state logic determiningif the last type of operation performed was from said processor; andlogic holding the row address strobe asserted if said last type ofoperation was from said processor and was of a predetermined processortype and otherwise setting the row address strobe negated if said lasttype of operation was from said processor and was not of saidpredetermined processor type.
 6. The computer system of claim 1, saidmemory controller further including:logic posting write operations fromsaid input/output bus; and logic determining if write operations areposted from said input/output bus, and wherein said predeternmined typeincludes said last type of operation being a write from said postingmeans and write operations are still posted.
 7. The computer system ofclaim 6, wherein said input/output bus includes read operationsindicating multiple read operations to occur and wherein saidpredetermined type includes the multiple read operation.
 8. The computersystem of claim 7, said memory controller further including:logicdetermining if the last type of operation performed was from saidprocessor; and logic holding the row address strobe asserted if saidlast type of operation was from said processor and was of apredetermined processor type and otherwise setting the row addressstrobe negated if said last type of operation was from said processorand was not of said predetermined processor type.
 9. The computer systemof claim 8, further comprising:a writeback cache system coupled to saidprocessor, and wherein said predetermined processor type includeswriteback cycles from said writeback cache.
 10. The computer system ofclaim 8, wherein said predetermined processor type of operation includesa processor code read.
 11. The computer system of claim 10, furthercomprising:a writeback cache system coupled to said processor andwherein said redetermined processor type includes writeback cycles fromsaid writeback cache.
 12. A method of operating a memory device whichincludes a row address strobe signal input, the method comprising thesteps of:completing a memory operation; determining if the memoryoperation is a memory operation of a predetermined type; determining ifthe memory operation has been initiated by a bus master on aninput/output bus; determining if the memory is in an idle state aftercompletion of the memory operation; and controlling the row addressstrobe signal input during the idle state by (1) asserting the rowaddress strobe signal predictive of a next operation requiring a pagehit to the memory device after the idle state if the memory operationwas from a bus master on an input/output bus and was the predeterminedtype as specified by control signals, or by (2) negating the row addressstrobe signal if the memory operation was from a bus master on theinput/output bus and was not the predetermined type of operation. 13.The method of claim 12 wherein the step of determining if the memoryoperation initiated by a bus master on an input/output bus comprisesdetermining if the memory operation has been initiated by a bus masteron a PCI bus.
 14. The method of claim 12 the predetermined type includesa multiple read operation.
 15. The method of claim 12 and furthercomprising the steps of:determining if the last type of operationperformed was from a processor; and holding the row address strobeasserted if the last type of operation was from said processor and wasof a predetermined processor type and otherwise setting the row addressstrobe negated if said last type of operation was from said processorand was not of said predetermined processor type.
 16. The method ofclaim 15 wherein the predetermined processor type of operation includeswriteback cycles from a writeback cache coupled to the processor. 17.The method of claim 15, wherein said predetermined processor type ofoperation includes a processor code read.
 18. A method of operating adigital computing system which includes a processor, a memory device andan input/output bus, the method comprising the steps of:applying a firstportion of a memory address to the memory device; applying a secondportion of a memory address to the memory device; performing a currentmemory operation, the current memory operation utilizing the first andsecond portions of the memory address; determining the type of operationperformed during the current memory operation; entering an idle stateduring which the memory device is not accessed; and predicting whether anext memory operation can utilize the first portion of the memoryaddress based on the type of operation performed during the currentmemory operation, the next memory operation occurring after the idlestate.
 19. The method of claim 18 and further comprising the step ofasserting a row address strobe signal if the predicting step predictsthat the next memory access can utilize the first portion of the memoryaddress.
 20. The method of claim 18 wherein the predicting step isfurther based on whether the current memory access was from a bus masteron the input/output bus.
 21. The method of claim 18 wherein the firstportion of the memory address is applied before the second portion ofthe memory address.
 22. The method of claim 18 and further comprisingthe step of refreshing the memory device.
 23. The method of claim 18 andfurther comprising the step of determining that next memory operationcan utilize the first portion of the memory address if a multiple readoperation was performed during the current memory operation.
 24. Themethod of claim 18 and further comprising the step of determining thatnext memory operation can utilize the first portion of the memoryaddress if the current memory operation included posting writeoperations from the input/output bus and write operations are stillposted.
 25. The method of claim 18 and further comprising the step ofdetermining that next memory operation can utilize the first portion ofthe memory address if the current memory operation was from theprocessor and was a writeback cycle from a writeback cache.
 26. Themethod of claim 18 and further comprising the step of determining thatnext memory operation can utilize the first portion of the memoryaddress if the current memory operation was from the processor and was aprocessor code read.